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The circuit described here
is that of a metal detector. The opera- tion of the circuit is based on superheterodyning
principle which is commonly used in superhet receivers. The circuit utilises
two RF oscillators. The frequencies of both oscillators are fixed at 5.5 MHz.
The first RF oscillator comprises transistor T1 (BF 494) and a 5.5MHz ceramic
filter commonly used in TV sound-IF section. The second oscillator is a Colpitt’s
oscillator realised with the help of transistor T3 (BF494) and inductor L1 (whose
construction details follow) shunted by trimmer capacitor VC1. These two oscillators’
frequencies (say Fx and Fy) are mixed in the mixer transistor T2 (another BF
494) and the difference or the beat frequency (Fx-Fy) output from collector
of transistor T2 is connected to detector stage comprising diodes D1 and D2
(both OA 79). The output is a pulsating DC which is passed through a low-pass
filter realised with the help of a 10k resistor R12 and two 15nF capacitors
C6 and C10. It is then passed to AF amplifier IC1 (2822M) via volume control
VR1 and the output is fed to an 8-ohm/1W speaker. The inductor L1 can be constructed
using 15 turns of 25SWG wire on a 10cm (4-inch) diameter air-core former and
then cementing it with insulating varnish. For proper operation of the circuit
it is critical that frequencies of both the oscillators are the same so as to
obtain zero beat in the absence of any metal in the near vicinity of the circuit.
The alignment of oscillator 2 (to match oscillator 1 frequency) can be done
with the help of trimmer capacitor VC1. When the two frequencies are equal,
the beat frequency is zero, i.e. beat frquency=Fx-Fy=0, and thus there is no
sound from the loudspeaker. When search coil L1 passes over metal, the metal
changes its inductance, thereby changing the second oscillator’s frequency.
So now Fx-Fy is not zero and the loudspeaker sounds. Thus one is able to detect
presence of metal |
